Energy storage devices such as a lithium-ion secondary battery are used as power supplies for mobile devices such as a notebook computer and a mobile phone, and nowadays used in a wide range of fields such as a power supply for an electric automobile. In such energy storage devices, it is necessary to correctly recognize the residual life that is of the usable accumulated residual period of the energy storage device. Conventionally, a technology of estimating a variation with time of a DC resistance of the energy storage device has been proposed in order to estimate the residual life of the energy storage device (for example, see Non-Patent Document 1).
Non-Patent Document 1 discloses a technology of estimating the variation with time of the DC resistance of a battery at a predetermined test time using an empirical formula in which the variation with time of the DC resistance is given by an N-order function of test time. Incidentally, N of the empirical formula is ½, 1, or 1.52, and changes according to a test mode.